The present invention generally relates to a control device for a hand-free telephone set. This device comprises an emission channel connected at one extremity to a microphone and at the other extremity to a telephone line interface and a reception channel connected at one extremity to the line interface and at the other extremity to a loudspeaker.
Such an apparatus comprises an amplifying loop constituted, on the one hand, by the two emission and reception channels and, on the other hand, by the loudspeaker/microphone acoustic coupling and by the electric coupling generated by the line interface which connects the emission and reception channels of each apparatus to the telephone line. If the gain of this amplifying loop is higher than one, an instability or Larsen effect occurs, which produces an unpleasant hiss.
In order to avoid this parasitic effect, an attenuator is generally disposed in each of the emission and reception channels.
An exemplary system, such as described for example in French patent application 87/10603 of July 21, 1987 is schematically shown in FIG. 1. The reception channel comprises a microphone R, an emission compressor GE, designed to supply at the output a compressed signal having a constant peak value, for example of about 100 millivolts, and an emission attenuator ATE, the output of which is connected to the telephone line L through a 2 wire/4 wire-type connection interface IL.
The reception channel similarly comprises a reception compressor GR and a reception attenuator ATR, the output of which is supplied to a loudspeaker HP. In order to avoid oscillations in the loop comprising the reception channel, the electric coupling AL between the line and the line interface, and the acoustic coupling AC between the loudspeaker and the microphone, each of the attenuators ATE and ATR is set to a minimum attenuation when the channel wherein it is positioned is active and to a maximum attenuation when the other channel is active. The system is said to be of the "alternate" type, that is, the talker cannot be interrupted by the party at the other end of the telephone line. In other words, it is not possible for those two parties to speak simultaneously since only one of the two channels of each telephone set is active, the emission channel of the talker and the reception channel of the listener.
The device described in the above-mentioned patent French application 87/10603 provides for an alternate control of the attenuators ATE and ATR by getting rid of the noise present in the microphone in the emission mode or in the line in the reception mode. Indeed, if the output signals of compressors GE and GR were compared for determining the operation of attenuators ATE and ATR, an important background noise in one of the channels (for example when one of the two parties speaks in a very noisy room) may generate a signal with an amplitude high enough at the output of the compressor for causing the attenuator of this channel to switch in active position (maximum attenuation). In such a case, there will always appear a maximum attenuation on the non-noisy channel and the other party could never be able to speak.
In order to avoid this drawback, the prior art device provides for detecting the peak value of the rectified signal at the output of each of the compressors GE and GR through peak detectors DE and DR. At the output of each of those peak detectors are provided circuits for detecting the noise signal, corresponding to integrators, IE and IR, respectively, having large time constants. Thus, it is possible to obtain at the input of a logic circuit CL information about the peak value of the signal on the emission channel (SE), the average value of the noise on the emission channel (SBE), the peak value of the signal on the reception channel (SR) and the average value of the noise on the reception channel (SBR). From those signals, the logic circuit CL supplies a first logic signal B/P indicating whether there is noise on both channels or whether someone is speaking on one of the channels, and a second logic signal E/R indicating, in case there is a speech signal, whether this speech signal has first appeared on the emission channel or on the reception channel. Those logic signals are supplied to a control circuit CC which, in presence of a speech logic signal on the output B/P, sets to low gain either emission or reception attenuator (ATE or ATR) corresponding to the channel on which a speech signal has been detected.
This prior art device supplies satisfactory results as regards selection of the active channel and elimination of noise signals but still presents some drawbacks during switching from one channel to another. Indeed, in such a device, as in the case of all known devices, attenuators operate in one of two states: a low or null predetermined attenuation state when the attenuator is in the active channel and in a predetermined maximum attenuation state when the attenuator is in the inactive channel, the transition between those two states, being ensured with a certain time constant. Moreover, some devices, such as the one of the above-mentioned patent application, provide for an intermediate attenuation position equal to half the maximum attenuation when none of the two channels is active, that is, when noise only is present on emission and reception channels.
The maximum attenuation of each of the attenuators being constant, said attenuation is to be chosen so as to be sufficient in the worst condition (that is, when the compressors have their maximum gain) and has therefore to be very high. As a result, during switching of the emission and reception channels, the transition is long if the switching time constants are chosen too high or causes an unpleasant parasitic effect for the listener.
Thus, an object of the invention is to palliate this drawback and reduce this adverse effect during switching without increasing the switching time constant of the system.
Another object of the invention is to further improve the logic circuit supplying the signals controlling the selection of attenuators.